1. Field of the Invention
The present invention relates to a variable displacement pump used in a pressure fluid utilization equipment such as a power steering apparatus for reducing a handle operating force of a vehicle.
2. Description of the Related Art
For example, a fluid pressure pump for use with a power steering apparatus is required to supply a full amount of pressure fluid to a power cylinder of a power steering apparatus to obtain a steering auxiliary force corresponding to a steering condition, when performing steering operation of a steering wheel (a so-called steering time). On the other hand, during the non-steering such as while the vehicle is running straight, supply of the pressure fluid is practically unnecessary. Also, the pump for the power steering apparatus is required to reduce the amount of supplying the pressure fluid while running at high speed below that at stoppage or while running at low speed, whereby it is desired to offer some stiffness to the steering wheel while running at high speed, and secure the driving stability while running straight at high speed.
Conventionally, the pump for the power steering apparatus of this kind is typically a displacement pump having an engine of the vehicle as a driving source. The displacement pump has a characteristic that the discharge flow is increased with greater number of rotations of the engine. Accordingly, when the displacement pump is employed as the pump for the power steering apparatus, a flow control valve is needed to control the discharge flow from the pump below a predetermined amount, irrespective of the number of rotations. However, with the displacement pump with the flow control valve, even if the pressure fluid is partially flowed back via the flow control valve to a tank, the load on the engine is not decreased, with an equal driving horse power of the pump, whereby the energy saving effect could not be obtained.
To resolve such a drawback, a variable displacement vane pump is conventionally proposed in which the discharge flow (cc/rev) per revolution of the pump can be decreased in proportion to an increase in the number of rotations, as described in JP-A-6-200883, JP-A-7-243385, and JP-A-8-200239. These variable displacement pumps are a so-called engine rotation number sensitive pump, in which if the engine rotation number (pump rotation number) is increased, the cam ring is moved in a direction where the pump displacement of the pump chamber is decreased, corresponding to the magnitude of a fluid pressure on the pump discharge side, so that the flow on the pump discharge side can be decreased.
The above variable displacement pump can increase the flow on the pump discharge side relatively when the engine rotation number is small at the stoppage or even while the vehicle is running at low speed, whereby the vehicle can gain a large steering auxiliary force in steering while the vehicle is stopped or running at low speed, and the driver can perform light steering. Also, while the vehicle is running at high speed, the engine rotation number is large, and the flow on the pump discharge side is relatively small, whereby the steering can be effected with an appropriate stiffness on the steering operation force while running at high speed.
Also, the variable displacement pump of this kind may supply a predetermined flow of pressure fluid at the time of steering (or when the steering is required) to obtain a predetermined steering auxiliary force, and the flow of pressure fluid as little as almost zero or the minimum as required during the non-steering (or while no steering is required), which is desired from the viewpoint of energy saving. For example, in a case where the variable displacement pump is directly driven by the engine of the vehicle, the discharge amount from the pump is unnecessary during the non-steering even when the engine rotation number is great. Then, by decreasing the pump discharge amount, the driving horse power of the pump can be suppressed, which respect should be desirably taken into consideration.
That is, in controlling the variable displacement pump of this kind, it is desired that the optimal pump control is performed by determining whether the vehicle is stopped, or running at low speed, medium speed or high speed, and whether the steering is made or not, and depending on the running condition of the vehicle. Accordingly, some measures must be taken in view of the operating condition of the pump and the running condition of the vehicle, so that the vehicle can exhibit the performance as the power steering apparatus by securely grasping the running condition and steering condition of the vehicle and appropriately making the pump control, and attain the energy saving effect as the variable displacement pump by making the driving control of the pump in a required condition.
The present invention has been achieved to solve the above-mentioned problems, and it is an object of the invention to provide a variable displacement pump in which while the vehicle is running straight, the pump discharge flow can be suppressed low, thereby improving the energy saving effect, and if it is needed to have a large flow at the time of steering, the variable displacement pump can respond quickly and increase the pump discharge flow to produce a required steering auxiliary force.
In order to accomplish the above object, according to a first aspect of the invention, there is provided a variable displacement pump comprising a cam ring supported slidably in an inner space of a pump body, a rotor disposed rotatably within the cam ring, a first fluid pressure chamber formed on one side of the cam ring, a second fluid pressure chamber formed on the other side thereof, biasing means for biasing the cam ring in a direction where the pump displacement of a pump chamber is at maximum, a metering orifice provided halfway on a discharge passage for supplying a pressure fluid discharged from the pump chamber to the pressure fluid utilization equipment, and a control valve for applying an upstream fluid pressure and a downstream fluid pressure of the metering orifice on both end faces of a spool, with a spring disposed on the side of an end face on which the downstream fluid pressure is applied, wherein the cam ring is swung by controlling at least one fluid pressure of the fluid pressure chamber through the activation of the control valve, characterized in that a piston that is moved with an increase in working pressure of the pressure fluid utilization equipment is provided to apply an axial thrust to an end face of the spool on the spring side.
According to a second aspect of the invention, there is provided the variable displacement pump, characterized in that the piston is a stepped piston disposed on the opposite side of the spool, with the spring interposed, one end of the spring contacted with a small diameter end of the piston, a working pressure of the pressure fluid utilization equipment applied on a large diameter end of the piston, whereby an axial thrust is applied via the spring to the spool of the control valve by introducing a lower pressure than the downstream fluid pressure of the metering orifice into a space formed around a step portion between a small diameter portion and a large diameter portion of the piston, and moving the piston by the use of a working pressure of the fluid pressure utilization equipment.
According to a third aspect of the invention, there is provided the variable displacement pump, characterized in that a second spring is disposed around the outer periphery of the spring, one end of the second spring being contacted with an end face of the spool, and the other end being contacted with an end face of a valve bore.
According to a fourth aspect of the invention, there is provided the variable displacement pump, characterized in that the piston is a stepped piston disposed on the opposite side of the spool, with the spring interposed, a working pressure of the pressure fluid utilization equipment applied on a large diameter end of the piston, a small diameter end extended to the spool side, wherein when the piston is moved by the use of a working pressure of the fluid pressure utilization equipment, an axial thrust is applied with a small diameter end of the piston directly contacted with the spool.
According to a fifth aspect of the invention, there is provided the variable displacement pump, characterized in that a change-over valve is provided halfway on an introduction passage for introducing a working pressure of the fluid pressure utilization equipment to a large diameter end of the piston, and when the working pressure is increased above a predetermined value, the change-over valve shuts off the introduction passage.